This invention relates to methods and apparatus for printing bead spacers on the substrates of liquid crystal display (LCD) panels.
LCDs are currently the most widely used type of flat panel display, and typically comprise a display panel having two substrates on which field generating electrodes are formed, and between which a layer of liquid crystal material is sealed. The panel adjusts the transmittance of light passing through the liquid crystal layer by rearranging the molecules of the liquid crystal layer through the application of selected voltages to the electrodes.
The upper and lower substrates of an LCD panel are coupled together with a sealant that extends around a peripheral edge of the two substrates, and which seals the liquid crystal material between them, and a plurality of spacers are disposed between the upper and lower substrates to define a space with a closely controlled height, called a “cell gap,” between the two substrates. The spacers may be comprise spherical “bead”-type spacers or “columnar”-type spacers that are disposed in a fixed pattern between the two substrates.
The columnar spacers are preferably formed in regions of the display panel through which the passage of light is blocked, for example, at the channel parts of thin film transistors (TFTs), gate lines, storage electrode lines, and the like, by coating a photosensitive film on a color filter array substrate of the panel and then exposing and developing it. The bead spacers, on the other hand, are typically formed by randomly spraying them onto one of the substrates before the two substrates are sandwiched together.
However, when bead spacers are simply sprayed randomly onto one of the display substrates, the spacers can act like particles of a foreign substance that adversely affect the contrast ratio of the display by allowing light to leak through them. Additionally, it is possible for some of the bead spacers to move slightly and thereby cause damage to an alignment layer of the panel.
On the other hand, the use of columnar-type spacers results in an increase in the number of manufacturing processes required to make the panel, thereby increasing panel cost. Additionally, unlike bead spacers, which are made of a plastic having a relatively high elasticity, columnar spacers have a relatively low elasticity, and as a result, the allowable tolerance in the amount of liquid crystal material needed to correctly fill the space between two the substrates decreases. As a result, it is easily possible to produce panels that are imperfectly or excessively charged with the liquid crystal material. Further, because of their low elasticity, when an excessive pressure is applied to columnar spacers, the display can easily develop a “smear” fault, in which the columnar spacers or the color filters disposed below them are broken.